Flat wire coils are known in the art. Such coils can for instance be used as the active component in linear motors. In such applications, a flat wire coil is typically wound from wire that has a substantially rectangular cross section. These wires are typically on the order of ten times thinner than they are wide.
Rectangular wires are interesting because the windings made from such wires stack better than round wire. A larger portion of the coil (by volume) is taken up by the conductor. Consequently, a coil having wire with a rectangular cross section typically shows a higher fill factor. This results in a lower resistance or a more compact design.
The wire to be used comprises a conductive core and an insulating jacket. The electric core conducts heat well, whereas the insulator conducts heat rather poorly. A higher fill factor allows the temperature of the coil to be lower allowing a more reliable and/or accurate operation.
In a linear motor, the flat wire coil is typically mounted on a cooling plate. The flat wire coil comprises a disc of wound wire, wherein the disc comprises a plurality of windings. The disc is mounted such that the windings lie against the cooling plate to ensure efficient cooling. This works best with a flat wire coil comprising a single layer. Alternatively, a flat wire coil having two layers or two flat wire coils each with a single layer mounted on top of each other may equally be used. Cooling may be performed on both sides.
For a given linear motor application one has to optimize the choice of motor as well as the power supply. A certain voltage, current and size of the coils of the motor will be decided on. The current together with the coil resistance determines how much energy is dissipated in the coil. The efficiency of the motor is typically above 90%, sometimes even 99%, but the generated heat still has to be transported away. Providing a heat conduction path to the environment is essential to keep the motor from burning out. Moreover, the thermal resistance combined with a given maximum operating temperature, determines the allowable current for the motor. Reducing the thermal resistance would increase motor performance (force) by allowing higher currents before the motor overheats.
To obtain a high thermal conductivity of the coil, the fill factor must be optimized. Given that some space in the coil is lost to the finite thickness of the insulator that surrounds the conductive core, a close packing of windings must be used to reduce this lost space. Geometrically, the optimum would be wire with a square cross section, because when filling up a rectangular area with many small shapes, rectangles are the most efficient, and the rectangle with the smallest circumference (which represents the insulator) is the square. However, the fill factor is not the only consideration.
Choosing a rectangle with a high aspect ratio gives the possibility to cross a significant part of the thickness of the coil with an unbroken copper “heat bridge”. In other words, the number of layers of insulator to cross is reduced for the heat to find its way out of the coil. However, the number cannot reach zero. There is always at least one layer of insulator between the conductor of the coil and the conductor of the motor housing.
So the engineering trade-off is between heat production and heat transport. The number of layers, combined with the thickness of the insulator and its thermal properties, yield an effective thermal resistance. The fill factor determines the heat dissipation. These two together determine the maximum continuous force the motor can generate while staying within a given specified temperature.
There are many industrial applications that require linear motion. Some of these require high accuracy, in the order of nanometers, with high accelerations and travel speed. Examples of such applications are pick-and-place machines and various applications in the semiconductor, solar panel and display manufacture industries. These motion requirements are suitably addressed by Linear Permanent Magnet Synchronous Motors (LPMSM).
Over time, more and more stringent requirements are placed on the linear motors. Thermal management becomes important for the following reasons. The continuous power output of a motor is ultimately limited by its ability to conduct heat out to an external heat sink. Furthermore, an uncontrolled heating up of any part of the construction leads to thermal expansion, which leads to positioning errors.
Through Ohmic dissipation, the coils are the main source of heat in a motor. At the same time, the largest thermal resistance is usually found in these same coils. For this reason, the traditional round wire coils are sometimes replaced by flat wire coils which combine lower heat dissipation with lower thermal resistance. This can be further optimized by choosing the number of layers in such a flat wire coil.
By increasing the number of windings in a coil, for instance by increasing the number of layers, the total amount of force to be exerted by the motor can be increased. However, flat wire coils of several layers are difficult to assemble with tight mechanical tolerances. Furthermore, when optimizing the fill factor, the insulator thickness is necessarily reduced which leads to enhanced risk of discharges between adjacently arranged layers. Additionally, when winding flat wire coils of a single layer, and then combining several of them in a stack, the number of process steps is high, and some of these carry a high risk of failure, such as soldering steps.